The Basics of a DC Circuit
Most personal watercraft have some form of electronic components that comprise the “house” systems. These include the lights, chart plotters, stereo system, etc. most of these systems are run on DC, or direct current, power. DC circuits are fairly simple at their core. They are composed of six pieces. A DC circuit needs a power source, an over-amperage control device, a conductor, a switch, a load, and a return path to ground. Without these six pieces a DC circuit will not function properly. Throughout this post we will go over the requirements of each of these six pieces.
Power Source/ Battery
This may be the most important part of the DC circuit. It is certainly the most common issue in a circuit. Batteries deteriorate, and when they do they stop producing voltage and amperage. Amperage is the current and voltage is the force that pushes that current. A circuit needs both to function properly. Batteries have different amperage ratings, which is an important consideration when we look at how long a component or series of components can be run on a battery, or battery bank. Additional considerations are the type and design of the battery. There are different chemistry types of batteries. Flooded cell, Absorbed Glass Matt (AGM), and Lithium-Ion. Each have their place, but AGM batteries are generally considered the best batteries for marine applications.
Beyond the chemistry of the design of the battery can impact the lifespan of the battery. There are starting/cranking batteries, and there are deep cycle batteries. Cranking batteries are designed to supply high amperage for short periods of time. The best example of this is the requirements of an engine to start. Cranking batteries suffer if they are drawn down routinely, which makes them a poor choice for applications like house batteries or trolling motor batteries. Deep cycle batteries are the opposite. They are designed to be used for lower amperage applications for longer times and can recover from depletion better. With that said, deep cycle batteries suffer from high amperage draws and will reduce the life of the battery. There is a third type of battery, which is called a dual-purpose, which doesn’t do either job very well and the lifespan of the battery suffers.
Over Amperage Control Device
There are a few types of over amperage control devices, but they all result in the same method of controlling amperage in a circuit. This method is to open the circuit. Some common devices are fuses and circuit breakers. A fuse reacts to over current by melting a specific length of conduit in a designated space to open a circuit. Circuit breakers flip open if they are exposed to too much heat , which is the primary output of too much current through a circuit. Circuit breakers can generally be reset, but there is wear and tear, and they can also fail. If a circuit breaker fails in the closed position it does not work to disrupt the circuit. Generally speaking, the overcurrent protection device should be as close to the power source as practical, to prevent damage from heat through the conductor.
The Conductor
The conductor is the means of transferring electrical current. This is the copper wire. Ideally this is tinned copper wire to prevent corrosion. Copper is the wire of choice for marine electrical because it is the most conducive metal for its price. The copper wiring needs to be insulated to prevent damage to the copper strands, and to prevent shorts to ground within the boat. A short to ground is guaranteed to set off the over current protection device, and if there is no protection device you risk a fire in your boat. At a minimum it will destroy your conductor if you lack the necessary protection.
The wire needs to be large enough to conduct the current needed to power the circuit. If the wire is not large enough it will build resistance and heat, and the circuit will fail.
A Switch
A circuit needs a means of turning the circuit on and off. This is done with some type of switch. For many components the switch is separate from the component itself. This is my most boats have some type of switch panel on the dash of the boat. It gives the operator the means of controlling components, like the horn, navigation lights, bilge pumps, or any number of other devices. Alternatively some components have their switches internal to the component. Some examples of this include gps units, radio head units, and pretty much anything with a screen. It is critical to be able to control if a component is on or off.
The Load
The load is the component, or device, that the circuit is powering. This can be something simple, or a more complicated component. With that said, all components still need power and ground to function. All components need a certain amount of voltage to function. simple circuits may start to dim before turning off, while other components will simply shut off, like chart plotters or engine control modules.
Return Path to Ground
All circuits need a return path to ground. A wise electrician I worked with once said “the world don’t go round without ground”, and he was right. After source power issues, ground is one of the leading issues in dc circuit problems. Diagnosing a bad ground does not have to be difficult, but identifying where the actual problem is can get very difficult.